1. Field of the Invention
The present invention generally relates to an electronic device and a method of manufacturing the same, and more particularly, to a device for radio-frequency modules in RF (radio frequency) systems that are used for wireless communication. With respect to the devices of this kind, there is an increasing demand for lighter, smaller devices with higher performance, smaller power consumption, and lower production costs. So as to obtain such devices, IPDs (Integrated Passive Devices) have become essential. An IPD is formed by integrating passive devices such as inductors and capacitors. The present invention relates to the structure of such an IPD and a method of manufacturing the IPD.
2. Description of the Related Art
Conventionally, passive devices are contained in or integrated with a substrate, so that the resultant device can be made smaller. In this manner, however, it is difficult to satisfy the demand for smaller devices that can be manufactured at lower production costs. For example, passive devices are formed between layers of a multi-layer substrates made of LTCC (low-temperature co-fired ceramic), and ICs or SAW filters are connected onto the layers with wires. For a smaller device, however, it is necessary to prepare a larger number of layers, and therefore, the production costs tend to increase, as well as the complexity in design. So as to eliminate this problem, attention is being drawn toward the development of an IPD in which passive devices are integrated by stacking thin films on a substrate made of ceramic, glass, silicon, or the like. For example, Harrier A. C. Timans, et al., disclose an IDP in “MEMS for wireless communications: ‘from RF-MEMS components to RF-MEMS-Sip’, IMEC vzw. Division Microsystems, Components and Packaging, 2003, pp. S139-S163”, in which lines and capacitors are formed on a glass substrate, and a dielectric layer (of a low-permittivity resin, such as BCB with E of 2.65) is formed over the lines and capacitors. A spiral inductor is formed on the resultant structure. Further, another dielectric material to cover the inductor is formed on the inductor, and pads on which wires and bumps are to be formed for connecting with the lines and other devices are prepared. Using an IPD chip manufactured by the above technique, it is possible to post-mount the device to a module in combination with a mounting technique such as SMT (surface mount technology), CSP (chip-scale package), SoC (system-on-chip), or SiP (system-in-a-package). It is also possible to mount ICs or SAW filters directly to the device. Accordingly, great decreases in production costs and sizes of the modules are expected. Harrier A. C. Timans, et al., also disclose various examples of RF modules on which IPDs are mounted. Other than Harrier A. C. Timans, et al., IDPs are disclosed in Japanese Unexamined Patent Publication Nos. 5-3404 and 4-61264 and U.S. Pat. No. 5,175,518.
With any of the above conventional structures, however, a large number of procedures and materials are required, resulting in high production costs. Therefore, it is difficult to produce less expensive devices in view of the use for low-cost modules to be built in mobile-phone handsets. Since the processing using a thick dielectric film is performed two or more times, and many procedures for removing metal films used in plating procedures are added to the manufacturing procedures, it is difficult to stabilize the processing conditions. The large number of layers also leads to a decrease in the reliability such as heat resistance of the device. Furthermore, as a thin, multi-layer dielectric film is used, the substrate is bent due to the difference in thermal expansion coefficient between the substrate and the dielectric film, if the substrate is thin. This hinders the use of a large-diameter substrate. In addition to that, as a thin-film metal film formed by sputtering or vapor deposition is used as part of the lines, the resistance becomes higher due to the skin effect caused by radio frequencies, and the device characteristics deteriorate.